1. Field of the Invention
This invention is concerned with a method for obtaining an electrical quantity corresponding to the specific enthalpy of steam, and a circuit arrangement for carrying out the method.
2. Description of the Prior Art
In thermal processes, a more accurate determination of the progress of the process can frequently be made from the enthalpy of the process than by only using the customary quantities of pressure and temperature. The three state quantities, pressure, temperature and enthalpy, have a large non-linear mutual relationship. Further because the enthalpy, i, is an unequivocal measure of the ratio of the heat input to the flow of the water through the system, the enthalpy, i, particularly in the control of boilers is highly suited as an auxiliary control quantity for the rapid sensing of heating disturbances. Up to now, however, the enthalpy, i, has not been used as a measurement or auxiliary control quantity, because its conversion into an electrical quantity having the required accuracy for control problems within an extended operating range and the permissible equipment expense, was considered as too difficult.
Empirically determined values of the enthalpy of water or steam as a function of temperature and pressure have been compiled in published tables (For example, the VDI Steam Tables) and are presented graphically in so-called (i,t) diagrams (See FIG. 1).
Also the enthalpy of steam has been expressed in algebraic form as a function of pressure and temperature, in the so-called Koch state equation.
The problem therefore is to provide a method by which the enthalpy of steam can be reproduced electrically by simple means with accuracy sufficient for control purposes over a relatively large range.
The range of interest is defined approximately as follows: EQU 50 kg/cm.sup.2 .ltoreq. p .ltoreq. 300 kg/cm.sup.2 EQU 250.degree.C .ltoreq. t .ltoreq. 450.degree.C EQU 600 kcal/kg .ltoreq. i .ltoreq. 750 kcal/kg
This operating range is entered in the (i,t) diagram in FIG. 1.